Summary Glioblastoma (GBM) is the most lethal primary brain cancer that is highly invasive and resistant to current treatments. Cancer invasion and tumor recurrence are universal in GBM patients despite maximal therapy. Some treatments such as anti-angiogenic regimens actually transform tumor growth towards a more invasive phenotype. Defining the mechanisms underlying the therapy-induced cancer invasion and tumor repopulation may help improve GBM treatment. GBMs contain abundant vessels consisting of endothelial cells (ECs) and pericytes. Pericytes play critical roles in maintaining vascular function and blood-brain (tumor) barriers. We found that the majority of vascular pericytes in GBMs are derived from glioma stem cells (GSCs), a highly plastic cellular subpopulation functionally defined by self-renewal, multipotency and tumor propagation. Selective targeting of GSC-derived pericytes disrupted tumor vessels and potently inhibited tumor growth, indicating that GSC-derived pericytes play crucial roles in supporting vascular structure and function to promote tumor growth. In addition, GSC-derived pericytes were detected on a subset of vessels in peritumoral brain, suggesting that these neoplastic pericytes have the capacity to migrate into brain tissues. Our recent study using cell lineage tracing demonstrated that GSC-derived pericytes have the potential to de-differentiate into GSCs that are able to grow tumors. Moreover, GSC-derived pericytes are highly resistant to current treatments, indicating that GSC-derived pericytes may function as a reservoir of tumor- initiating cells to promote cancer invasion and tumor recurrence by de-differentiating into GSCs after therapy. Thus, we hypothesize that GSC-derived pericytes are reserve cancer cells that contribute to the therapy-induced cancer invasion and tumor recurrence and targeting these neoplastic pericytes synergizes with current GBM treatments. We plan to test our hypothesis by pursuing three specific aims: 1. Determine roles of GSC-derived pericytes in the therapy-induced cancer invasion and tumor recurrence. 2. Define the molecular mechanisms driving de-differentiation of GSC-derived pericytes into GSCs. 3. Determine whether targeting GSC-derived pericytes synergizes with current GBM therapies. We will use in vivo cell lineage tracing, selective targeting and other new techniques to elucidate the roles of GSC-derived pericytes in the therapy-induced cancer progression and evaluate the synergistic impact of targeting GSC-derived pericytes with current therapies in a preclinical setting. The proposed studies will lay a solid foundation for the development of effective therapeutics or new treatment combinations to significantly improve survival of GBM patients.